Shamail Tirmidhi

It is narrated from Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him), he says that the Noble Prophet (peace and blessings be upon him) would perform the Duha (forenoon) prayer with such regularity that we would say, "He will never abandon it now," and sometimes he would leave it with such regularity that we would say, "He will not perform it now."