Masarrah bin Ma'bad al-Lakhmi (may Allah have mercy on him) relates that Yazid bin Abi Kabshah (may Allah have mercy on him) led us in the Asr prayer, then after giving the salam he turned towards us and informed us that he had prayed behind Marwan bin al-Hakam (may Allah have mercy on him), and he made us perform prostrations like these two prostrations. Then Marwan bin al-Hakam (may Allah have mercy on him) said: "Indeed, I prayed behind Sayyiduna Uthman bin Affan (may Allah be pleased with him), and he made us perform prostrations like these two prostrations." Then Sayyiduna Uthman bin Affan (may Allah be pleased with him) said: "I was with your Prophet (peace and blessings be upon him) when a man came to him and said: O Prophet of Allah! I prayed but could not tell whether I had prayed an even number of rak'ahs or an odd number. Then I prayed but could not tell whether I had prayed an even number or an odd number. He said this three times. So the Prophet of Allah (peace and blessings be upon him) answered him, and the Prophet of Allah (peace and blessings be upon him) said: 'Satan plays with you in your prayer; so whoever prays and cannot tell whether he has prayed an even number or an odd number, let him perform two prostrations, for they are the completion of his prayer.'"
No one other than Sawwar bin Umarah narrated this hadith from Masarrah bin Ma'bad, and this report is narrated from Sayyiduna Uthman (may Allah be pleased with him) only with this chain.
Hadith Referenceالمعجم الأوسط / باب العين / 4700
Hadith Takhrij«أخرجه الضياء المقدسي فى «الأحاديث المختارة» برقم: 386، وأحمد فى «مسنده» برقم: 457، 458»