The Quran

وَيَقولونَ طاعَةٌ فَإِذا بَرَزوا مِن عِندِكَ بَيَّتَ طائِفَةٌ مِنهُم غَيرَ الَّذي تَقولُ ۖ وَاللَّهُ يَكتُبُ ما يُبَيِّتونَ ۖ فَأَعرِض عَنهُم وَتَوَكَّل عَلَى اللَّهِ ۚ وَكَفىٰ بِاللَّهِ وَكيلًا

Muhammad Asad

And they say, "We do pay heed unto thee"¹ - but when they leave thy presence, some of them devise, in the dark of night, [beliefs] other than thou art voicing;² and all the while God records what they thus devise in the dark of night. Leave them, then, alone, and place thy trust in God: for none is as worthy of trust as God.

Lit., "And they say, 'Obedience'" - a reference to the hypocrites of Medina, in the time of the Prophet, and - by implication - the hypocritical "admirers" and half-hearted followers of Islam at all times.
I.e., they surreptitiously try to corrupt the message of God's Apostle. The verb bata denotes "he spent the night"; in the form bayyata it signifies "he meditated by night [upon something, or upon doing something]", or "he devised (something) by night" (Lisan al-'Arab), i.e., in secrecy, which is symbolized by "the dark of night".