Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 152
Narated By Al-Mughirah b. Shu'bah : The Messenger of Allah (may peace bç upon him) lagged behind (in a journey). He then narrated this story saying : Then we came to the people. 'Abd al-Rahman was leading thorn in the dawn prayer. When he perceived the presence of the Prophet (may peace be upon him), he intended to retire. The Prophet (may peace be upon him) asked him to continue and I and the Prophet (may peace be upon him) offered one rak'ah of prayer behind him. When he had pronounced the salutation, the Prophet (may peace be upon him) got up and offered the rak'ah which had been finished before, and he made no addition to it.
Abu Dawud said: Abu Sa'id al-Khudri, Ibn al-Zubair and Ibn 'Umar hold the opinion that whoever gets an odd number of the rak'ahs of prayer, he should perform two prostrations on account of forgetfulness.