Firstly: 

It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “Allaah says: ‘I have prepared for My righteous slaves that which no eye has seen, no ear has heard and it has never crossed the mind of man. All of that is reserved, besides which all that you have known is as nothing.’ Then he recited: ‘No person knows what is kept hidden for them of joy as a reward for what they used to do’ [al-Sajdah 32:17 – interpretation of the meaning].” 

Narrated by al-Bukhaari (3072) and Muslim (2824) -- the report narrated by Muslim adds: “apart from that of which Allaah has informed you.” 

Secondly: 

With regard to the Prophet (blessings and peace of Allah be upon him) seeing Paradise and entering it, that is proven in many hadeeths. Imam al-Bukhaari, in his book al-Sharee‘ah, quoted some of these hadeeths under the chapter heading Baab Dukhool al-Nabi (sall Allahu ‘alayhi wa sallam) al-Jannah (The Prophet’s entering Paradise) in which he narrated several hadeeths, including the following: 

It was narrated from Anas ibn Maalik that the Prophet (blessings and peace of Allah be upon him) said: Whilst I was walking in Paradise, I saw a river whose banks were domes of hollow pearls.” I said, “What is this, O Jibreel?” He said: “This is al-Kawthar which your Lord has given to you.” The angel struck it with his hand and its mud or its perfume was of the most fragrant (or pure) musk.  

Narrated by al-Bukhaari (6201). 

According to another report: When the Prophet (blessings and peace of Allah be upon him) was taken up to heaven (the Mi‘raaj), [he said:] I came to a river whose banks … 

According to a version narrated in Musnad Ahmad (19/66): “I entered Paradise and saw a river whose banks were pavilions of pearls.” 

Thirdly: 

With regard to the issue mentioned by the questioner, it may be said: This that has been prepared in Paradise for the righteous slaves of Allah has not been seen by anyone, as it states in the texts. As for what Adam and our Prophet Muhammad (blessings and peace of Allah be upon them) saw, it was only part of in Paradise because Paradise, “in which there is that which no eye has seen” is not necessarily that which no one has ever seen. Rather it may be that there are in it things that some eyes have seen, even though there is in it that which is stored up and hidden, no eye has seen it and no ear has heard, and no one can imagine it or describe it. 

Perhaps the above meaning is supported by the hadeeth of al-Mugheerah ibn Shu‘bah, who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Moosa asked his Lord: ‘Who will be the lowest of the people of Paradise in status?’ He said: ‘He will be a man who will come after I have admitted the people of Paradise to Paradise and it will be said to him: “Enter Paradise.” He will say: “O Lord, how, when the people have taken their places and been given their portions?” It will be said to him: “Would it please you if you had the like of what one of the kings of the world had?” He will say: “I would be pleased, O Lord.” He will say, “You will have that, and as much again, and as much again, and as much again, and as much again.” The fifth time, he will say, “I am pleased, O Lord.” He will say: “You will have that and ten times as much, and you will have what your heart desires and what will delight your eyes.” He will say: “I am pleased, O Lord.”’ (Moosa) said: ‘My Lord, who will be the highest of them in status?’ He said, ‘They will be the ones whom I have chosen, and I have planted their honour with My own hand. I have set a seal over it and no eye has seen, no ear has heard, nor has it entered the heart of man.’” He said: And the confirmation thereof is in the Book of Allaah, may He be glorified and exalted: “No person knows what is kept hidden for them of joy as a reward for what they used to do” [al-Sajdah 32:17]. 

Narrated by Muslim, 189. 

It is as if those who are highest in status are those who have the greatest share of this hidden treasure and blessing that no eye has seen and no mind has imagined. 

And Allah knows best.