daf bit: Chullin 88

Raba taught: as a reward for our father Avraham having said "I am but dust and ashes" his descendants were worthy to receive two commandments: the ashes of the red heifer (which remove ritual impurity), and the dust used in the ritual of a woman accused of adultery (sotah). On account of his saying "I will not take even a thread or a sandal-strap" (as spoils of war) his descendants were worthy to receive two more commandments: the thread of blue in the tzitzit (fringes), and the strap of the t'fillin (phylacteries). And R. Meir further expounds: why was blue chosen for the thread in the tzitzit? Because blue resembles the color of the sea, the sea resembles the sky, the sky resembles a sapphire, and a sapphire resembles the throne of glory. (88b-89a)

I don't know if there's any significance to the fact that the rabbis here identify two of the rarest mitzvot (the red-heifer ritual was rare even when there was a temple) and two of the more common ones (tzitzit and t'fillin are daily commandments).