Sayyiduna Abd al-Rahman bin Awf (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "When any of you is in doubt about a deficiency in his prayer, let him pray on until his doubt is about an addition."
The like of it has already passed in the hadith of Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him), Sayyiduna Ibn Umar (may Allah be pleased with them both) and Sayyiduna Anas bin Malik (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3858