I heard that the Gra brings a raya from the Gemara in Kesubos (5a) that the mitzva is only by day. The Gemara says:
?”? ???? ??? ???? ??? ??????? ??? ????? ???? ???? ???? ????? ??? ????? ?? ??? … ??? ??? ??? ????? ??? ??? ??? ?????
And Rashi explains:
??? ??? ????? – ????? ????? ??????? ???? ?? ????
??? ??? ??? ????? – ??????? ????? ??? ???? ??????? ???
Btw I would just like to give public recognition to ???????? from which I copy and paste most times I wish to quote from Shas. It is an invaluable resource.