Parashas Bo · Passuk 12:22
Chabad.org — English translation + Rashi
Torah Temimah points to Mechilta (verified).
Sefaria — Exodus 12:22
Sefaria — Mechilta
English: “An agudah of hyssop”: from this the Sages derive that other Torah acts of taking specified without further qualification likewise involve a bundle; the required hyssop is ordinary hyssop, not varieties bearing an additional designation.
In context: See the cited source for the full section.
Derash defining both the bundled form of the hyssop and the qualifying species.
The hyssop used for the Passover blood must be an ordinary qualifying hyssop and taken as a bundle.