What is the source in the Halachic literature outside of the Mishnah Berurah who says it is a requirement to place a washing cup by your bed instead of going to the sink to wash your hands upon awakening in the morning?
The Mishna Brura (OC 1 sk 2) quotes the Shaarei Teshuva (OC 1 sk 2) who quotes the Birkei Yosef (OC 1 sk 1) who quotes the Bach (OC 4 sk 1) who quotes the Tolaat Yaakov who quotes the Zohar that it is assur to walk 4 cubits before washing upon waking up. It would thus seem prudent to keep a cup of water by one's bed.
Unfortunately, we don't have a Zohar that says that. So I give you: the Tolaat Yaakov!
h/t @simchashatorah for this bio of the Tolaat Yaakov.
Kaf HaHaim Palagi (8:1) writes that if someone doesn't have the cup around his bed, then he should walk four amot and then stop etc. Thus, he holds that one should water around his bed.