A.T. Robertson Commentary Luke 9:18

"And it came to pass, as he was praying apart, the disciples were with him: and he asked them, saying, Who do the multitudes say that I am?" — Luke 9:18 (ASV)

As he was praying (εν τω εινα αυτον προσευχομενον). Common Lukan idiom of εν with the articular infinitive for a temporal clause, only here Luke has the periphrastic infinitive (εινα προσευχομενον) as also in 11:1. This item about Christ's praying alone in Luke.

Alone (κατα μονας). In the N.T. only here and Mr 4:10. Perhaps χωρας (places) is to be supplied with μονας (lonely places).

Were with him (συνησαν αυτω). This seems like a contradiction unless "alone" is to be taken with συνησαν. Westcott and Hort put συνηντησαν in the margin. This would mean that as Jesus was praying alone, the disciples fell in with him. At any rate he was praying apart from them.