A.T. Robertson Commentary Luke 6:12

"And it came to pass in these days, that he went out into the mountain to pray; and he continued all night in prayer to God." — Luke 6:12 (ASV)

He went out into the mountains to pray (εξελθειν αυτον εις το ορος προσευξασθα). Note εξ- where Mr 3:13 has

goeth up (αναβαινε). Luke alone has "to pray" as he so often notes the habit of prayer in Jesus.

He continued all night (ην διανυκτερευων). Periphrastic imperfect active. Here alone in the N.T., but common in the LXX and in late Greek writers. Medical writers used it of whole night vigils.

In prayer to God (εν τη προσευχη του θεου). Objective genitive του θεου. This phrase occurs nowhere else. Προσευχη does not mean "place of prayer" or synagogue as in Ac 16:13, but the actual prayer of Jesus to the Father all night long. He needed the Father's guidance now in the choice of the Apostles in the morning.