Matt 6:5 οτι

The optional conjunction οτι appears after αμην λεγω υμιν in most witnesses (including E K L M S U W Δ Θ Π Σ Φ Ω 565 579 1241 1424 f; Or-pt Bas-pt) but not in a few others (ℵ Β D Z 047 f1.13 28 33 700 pc lat; Or-pt Bas-pt Chr). Griesbach (1:66–7) reasons, "Some omit οτι both here and in vs. 16, because it is not employed in vs. 2." Von Soden similarly, but printing οτι within brackets, writes, "In 6:5, 16 the omission of οτι after υμιν (cf. 6:2) is to be judged in the same way [as the omission of αν {6:5}, which is secondary, being conformed to 6:2]" (1:1424).
     The expression αμην λεγω υμιν appears 25 times in Matthew, seven uncontested with οτι following (18:13, 18; 19:23, 28; 21:31; 24:47; 26:21) and 13 uncontested without οτι (6:2; 8:10; 10:15, 42; 11:11; 18:3, 18; 21:21; 24:2; 25:12, 40, 45; 26:13). The three contested places where RP adds and NA27 omits οτι are: 6:5, 16; 23:36. The two contested places where RP omits and NA27 adds οτι are: 16:28; 24:34.
     In the present unit of variation the internal argument of Griesbach and von Soden is sensible. As in Matt 6:4 αυτος, Matt 6:4 εν τω φανερω, and Matt 6:5 αν, the less harmonized reading is generally preferable to the more harmonized one. In 6:16 the witnesses that omit οτι (ℵ B D 0233 0250 f1.13 565 700 al it) are similar to the ones that do so here. Thus in a few largely related manuscripts the second and third appearances of the expression in 6:5, 16 (αμην λεγω υμιν οτι απεχουσιν τον μισθον αυτων) were conformed to the first in 6:2 (αμην λεγω υμιν απεχουσιν τον μισθον αυτων), i.e., without οτι. This internal factor combined with the observation that οτι is completely unnecessary sufficiently explains why it came to be omitted here in some witnesses. Cf. also the note on Matt 5:31 οτι.

Other cases of omission of οτι in a minority of witnesses in Matthew:
5:32 (D 1506 it; Aug)
9:18 (ℵ D M f1.13 33 205 892 pc lat)
10:7 (B 1506 sy-s)
11:24 (ℵ*.2 33 pc; Ir-lat)
13:11 (M pc)
16:17 (B* 1424*)
16:18 (Θ 1 1506 1582 ff2 g1; Eus Chr Phot)
19:9 (B D Z 1424 pc it)
20:12 (ℵ B C2 D 085 f1 33vid 1424 lat sy-s.c.p; Or Chr)
21:16 (ℵ D S 983 1506 pc it vg-mss; Ir-lat)
21:43 (ℵ B* Θ 205 209 565 700 892 1506 pc)
23:36 (p77 ℵ B D L Θ f1.13 205 579 700 892* pc it)
26:29 (p45 ℵ B D Z Θ 0281 f1.13 33 205 892* al; Ir-lat)
26:65 (ℵ2 B C2 D L Z Θ 0281 33 700 892 pc latt; Or Cyr Chr)
26:72 (ℵ D pc)
26:75 (D pc it)
27:47 (ℵ D L Θ 33 700 892 1424 pc lat sy-s.p)
28:13 (33)  

Cases of addition of οτι in a minority of witnesses in Matthew:
6:2 (Zvid 157 pc h)
8:25 (Δ)
9:33 (V Θ 35 1424)
12:23 (D*)
14:4 (M pc)
16:28 (ℵ B L Θ f13 33 579 700 1424 al it)
17:11 (ℵ)
17:20 (C pc co; Or)
19:24 (ℵ C L M Z 0281 579 892 al aur)
21:11 (N)
23:39 (D Θ f1.13 205 it; Or)
24:2 (D sy-p)
24:5 (C* Φ pc f; Or)
24:34 (B D F L Θ f1.13 33 205 700 892 1424 al latt; Or)
26:64 (D pc bo)
27:63 (D 157 pc; Chr Or-lat)
27:64 (33 al a ff2 sy-s.p)