Luke 22:43-44

(Luke 22:43-44) Then an angel from heaven appeared to him and strengthened him. But getting into an agony he continued praying more earnestly; and his sweat became as drops of blood falling to the ground.

According to Comfort's Textual Commentary, "the manuscript evidence for this textual variant is decidedly in favor of the exclusion of 22:43-44."  Some of the manuscripts that omit these Lukan passages include "P69vid, P75, Sinaiticus1, A, B, N, T, W, etc. (see Comfort)

Further, the NET Bible footnote states:

Several important Greek mss (Ì⁷⁵ א¹ A B N T W 579 1071*) along with diverse and widespread versional witnesses lack 22:43-44. In addition, the verses are placed after Matt 26:39 by Ë¹³. Floating texts typically suggest both spuriousness and early scribal impulses to regard the verses as historically authentic. These verses are included in א*^,2 D L Θ Ψ 0171 Ë¹ Ï lat Ju Ir Hipp Eus. However, a number of mss mark the text with an asterisk or obelisk, indicating the scribe’s assessment of the verses as inauthentic. At the same time, these verses generally fit Luke’s style. Arguments can be given on both sides about whether scribes would tend to include or omit such comments about Jesus’ humanity and an angel’s help. But even if the verses are not literarily authentic, they are probably historically authentic. This is due to the fact that this text was well known in several different locales from a very early period. Since there are no synoptic parallels to this account and since there is no obvious reason for adding these words here, it is very likely that such verses recount a part of the actual suffering of our Lord. Nevertheless, because of the serious doubts as to these verses’ authenticity, they have been put in brackets. For an important discussion of this problem, see B. D. Ehrman and M. A. Plunkett, “The Angel and the Agony: The Textual Problem of Luke 22:43-44,” CBQ 45 (1983): 401-16.

Given all the above evidence, we should view these passages as interpolations and not original. Luke didn't pen these verses. Though it should be noted that many regard these passages as "historically true," that is, that it did actually happen but that Luke for some reasons didn't include it in his Gospel. I wouldn't accept that uncritically, however.

(see also the New World Translation with References' footnote on Luke 22:44)