Professional Prayer

Rashi (14:10) explains ויצקעו בנ"י אל ה' as תפסו אומנות אבותם בידם.  Why does Rashi call their prayer an אומנות and what is he explaining, we already encountered prayers of Bnei Yisroel in previous chapters?  The Rebbe explains (Likutay Sichos volume 11) that the difficulty in the verse is why Bnei Yisroel were praying.  They were already promised to be brought to Eretz Yisroel, so if they believed, then there is no danger, and if they don’t believe, then why bother to pray?  Rashi answers that yes, they believed.  They weren’t praying because of the danger in front of them, rather they prayed because that was there practice, their אומנות.  Rashi cites proof from pesukim of the avos where Chazal derive that they prayed.  Why didn’t he cite open pessukim where we find the avos prayed?  Because that is a prayer out of need, Rashi is proving the prayer of אומנות, that it was a regular activity and the proof of that is a derash.