Can he say du ‘aa’ between the two prostrations in words other than those narrated from the Prophet (blessings and peace of Allah be upon him)?

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Check below answers in case you are looking for other related questions:
Ruling on the takbeers of movement (in the prayer) and the du‘aa’ between the two prostrations
Ruling on counting the number of rak‘ahs on the fingers whilst praying

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Related Answers:
He has the problem of yawning a great deal whilst praying – what should he do?
He assumed that he had done less in his prayer, but he only did the two prostrations of forgetfulness without doing the rak‘ahs that he had omitted
Naafil prayers that it is permissible to do at times when prayer is otherwise forbidden
Ruling on the takbeers of movement (in the prayer) and the du‘aa’ between the two prostrations
Ruling on counting the number of rak‘ahs on the fingers whilst praying

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