John Gill Commentary Acts 5:9

"But Peter [said] unto her, How is it that ye have agreed together to try the Spirit of the Lord? behold, the feet of them that have buried thy husband are at the door, and they shall carry thee out." — Acts 5:9 (ASV)

Then Peter said to her, how is it that you have agreed
together
For husband and wife to agree together in what is good, in things civil, honest, and lawful, and in religious matters, is very commendable; but to agree in a fraud, in a lie, is very dreadful:

to tempt the Spirit of the Lord ;
to try whether the apostles had the Spirit of the Lord, or not; and whether the Spirit of the Lord that was in them was omniscient and omnipotent, would take any notice of it, and inflict punishment for it:

behold, the feet of them which have buried your husband are at the
door ;
which Peter knew either by hearing the sound of their feet, as Ahijah the prophet heard the sound of the feet of Jeroboam's wife, as she came in at the door, (1 Kings 14:6) or by the same spirit as Elisha knew that Gehazi ran after Naaman, and received money and garments from him, (2 Kings 5:26) and shall carry you out; of this house dead, and bury you, as they have your husband.