Commentary Purg XXXI 46

Tommaseo (DDP Tommaseo.Purg.XXXI.46-48) was apparently the first to explain the strange phrase 'to sow tears' by referring to the Bible (Psalms 125:5 [126:5]), 'those that sow in tears shall reap in joy.'  Even so, Dante has made the Psalmist more difficult than he had in fact been: 'sowing tears' is not quite the same thing as 'sowing in tears' (i.e., planting seed while weeping).