Ishaq bin Sa'id bin Amr bin Sa'id bin al-As (may Allah have mercy on him) narrates: My father narrated to me from his father, who said: I was in the presence of Sayyiduna Uthman (may Allah be pleased with him). He called for water for ablution and said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Any Muslim upon whom the time of an obligatory prayer comes, and he performs its ablution well, and performs its humility and devotion and its bowing well, it will be an expiation for his previous sins, as long as he does not commit a major sin; and this holds for all time."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3582