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
It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that once the Messenger of Allah (peace and blessings be upon him) led the prayer, then said: "O so-and-so! Why do you not perform your prayer well? Does the one who prays not consider, when he prays, how he is praying? He prays only for his own benefit. By Allah! I see what is behind me just as I see what is in front of me."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3583
It is narrated from Sayyiduna Abdullah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever performs the prayer well in order to show people, and does not perform it well when alone, that is an act of contempt and mockery towards his Lord."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3584
Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) came out and said: "O people! Beware of hidden shirk." The Companions (may Allah be pleased with them) said: O Messenger of Allah! What is hidden shirk? He (peace and blessings be upon him) said: "That a man stands for prayer and strives to beautify his prayer so that people may see him; that is hidden shirk."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3585
It is narrated from Sayyiduna Salman al-Farisi (may Allah be pleased with him): "Prayer is a measure. Whoever fulfils it completely will be given his reward in full, and whoever falls short, you know well what has been said about those who give short measure."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3586