It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that we were discussing some matter of the prayer with Sayyiduna Umar (may Allah be pleased with him) when Sayyiduna Abdur Rahman bin Awf (may Allah be pleased with him) came to us and said: "Shall I not narrate to you a hadith that I heard from the Messenger of Allah (peace and blessings be upon him)?" We said: Certainly! Please do. He said: I bear witness that I certainly heard this hadith from the Messenger of Allah (peace and blessings be upon him): "When one of you has a doubt about falling short in his prayer, let him pray in such a way that his doubt becomes about excess."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3807