Jabir bin Abdullah (may Allah be pleased with them both) narrates: the Prophet (peace and blessings be upon him) sent me on an errand of his (own), and I returned having completed it. I greeted him (peace and blessings be upon him), but he did not reply. At that time he (peace and blessings be upon him) was praying.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3347