A.T. Robertson Commentary John 4:37

"For herein is the saying true, One soweth, and another reapeth." — John 4:37 (ASV)

For herein (εν γαρ τουτω). In this relation between the sower and the reaper.

The saying (ο λογος). Like 1 Timothy 1:15; 3:1, etc. Probably a proverb that is particularly true (αληθινος for which see 1:9) in the spiritual realm.

One soweth, and another reapeth (αλλος εστιν ο σπειρων κα αλλος ο θεριζων). "One is the sower and another the reaper." It is sad when the sower misses the joy of reaping (Job 31:8) and has only the sowing in tears (Psalms 126:5f.). This may be the punishment for sin (Deuteronomy 28:30; Micah 6:15). Sometimes one reaps where he has not sown (Deuteronomy 6:11; Joshua 24:13). It is the prerogative of the Master to reap (Matthew 25:26f.), but Jesus here lets the disciples share his joy.