וַיָּבֹא הָאִישׁ הַבַּיְתָה וַיְפַתַּח הַגְּמַלִּים וַיִּתֵּן תֶּבֶן וּמִסְפּוֹא לַגְּמַלִּים וּמַיִם לִרְחֹץ רַגְלָיו וְרַגְלֵי הָאֲנָשִׁים אֲשֶׁר אִתּוֹ.
Breishis 24:32
Rashi on this Posuk says that he loosened their muzzles, for he had sealed their mouths, so they (the camels) should not graze in other people's fields. The Ramban among many others asks a question on this Pshat. The Gemara says that the donkey of Reb Pinchas Ben Yair would not eat from non-Tithed (not taken Masser from) crops, so why is it that Avraham is concerned about his camels?