'O 'Abd Allah, when you offer salat, pray like someone who bids farewell and fears that he will never return (i.e. pray in such a manner as if it were the last prayer of your life). Then fix your gaze on the point of your prostration. If you know that there is someone on your left or right, you take more care in offering your salat; then know that you stand in front of Someone Who sees you and you don't see Him.'
Al-Hurr al-`Amili, Wasa'il al-Shi'ah, iv, 688.
Al-Shaykh al-Saduq-may God sanctify his soul-reports with his isnad from al-'Imam al-Sadiq-may peace be upon him-that he said:
'When someone offers an obligatory salat at the beginning of its time and observes its prerequisites, an angel takes it up to the heaven. White and pure, it (i.e. the salat) says, 'May God take care of you as you took care of me. I have been delivered into the custody of a noble angel.' But when someone offers it without any excuse after its time has elapsed and does not observe its requisite manners, black and dark, it is taken up by an angel, while it calls out to him (i.e. its offerer), `You neglected me. May God neglect you in the same manner that you neglected me. May God not take care of you in the same way that you did not take care of me.'
Wasa'il al-Shi`ah, iii, 90, with a slight difference of wording.