Silsilah as-Sahihah 682 — Different forms of prostration

It is narrated from Sayyiduna Abdur Rahman bin Awf (may Allah be pleased with him) that he said: The Noble Prophet (peace and blessings of Allah be upon him) said: "When a person forgets in prayer and does not know whether he has prayed one rak‘ah or two, he should base his prayer on one. And when he does not know whether he has prayed two or three, he should base it on two. Similarly, when he is in doubt between three and four, he should consider three as certain, and (after completing the prayer accordingly) perform two prostrations before the salutation."