prostrate at that very moment—that is, your turning among them, meaning: together with them—for Allah, glorified and exalted, observes a person both when he stands and when he prostrates.
Standing and prostration are singled out here because, within the prayer, standing surpasses prostration in terms of the words recited, whereas prostration surpasses standing in terms of physical posture.
Standing is superior in its recitation, for the prescribed remembrance during standing is the recitation of the Quran, and the Quran is the noblest of speech.
Prostration, however, is nobler in posture, for the worshipper in prostration is at his closest to his Lord. The Prophet (peace and blessings be upon him) affirmed this, saying, “The closest a servant is to his Lord is while he is in prostration.”
For that reason we have been instructed to make abundant supplication during prostration.
Another aspect of your vigilance toward Allah is to know that He hears you. Whatever you say, Allah, Exalted is He, hears it, as He says: “Or do they think We do not hear their secrets and private counsel? Indeed We do, and Our messengers are with them recording” (al-Zukhruf 80).
Moreover, whatever you utter—good or evil, in secret or in public—is recorded for you or against you, as Allah, blessed and exalted, says: “Not a word does he utter but there is with him a watchful, ready recorder” (Qāf 18). Therefore, be ever-watchful of this matter and beware of letting your tongue release a word for which you will be brought to account on the Day of Resurrection. Keep your tongue speaking the truth, or else remain silent, just as the Prophet (peace and blessings be upon him) said: “Whoever believes in Allah and the Last Day—”