Sefer HaBahir 25

1Rabbi Berachyah said, “Why is it written, ‘And Elohim said, ‘Let there be light,’ and there was [ויהי] light’ (Genesis 1:3), and not, ‘And there was [והיה] light’? A parable: a king that had a nice object and set it aside until he prepared for it a place and put it there. As it is written, 'Let there be light, and there was light' — that it already was [היה]."