A.T. Robertson Commentary John 1:40

"One of the two that heard John [speak], and followed him, was Andrew, Simon Peter`s brother." — John 1:40 (ASV)

Andrew (Ανδρεας). Explained by John as one of the two disciples of the Baptist and identified as the brother of the famous Simon Peter (cf. also 6:8; 12:22). The more formal call of Andrew and Simon, James and John, comes later (Mr 1:16ff.; Matthew 4:18ff.; Luke 3:1–11).

That heard John speak (των ακουσαντων παρα Ιωανου). "That heard from John," a classical idiom (παρα with ablative after ακουω) seen also in 6:45; 7:51; 8:26,40; 15:15.