On His Blindness is a poem by John Milton.
In 2370, after sending William Riker, Kira Nerys, Data, Odo and Deanna Troi on a mission into the Gamma Quadrant to search for a missing runabout, both Jean-Luc Picard and Benjamin Sisko recalled the final line of the poem, "They also serve who only stand and wait". Sisko asked Picard, who often repeated the phrase in such situations, if it helped. Picard noted it did not. (TNG - DS9 comic: "Prophets and Losses")