LSJ
for ἀνά, before words beginning with β, π, φ, μ, e.g. ἂμ βωμοῖσι, ἂμ μέσον, ἂμ πεδίον, ἂμ πέλαγος, ἂμ φυτά; also compds., as ἀμπαύω ; — mostly Dor., as in Pi., but also in Hom., sts. in Trag.
Liddell-Scott-Jones, Greek-English Lexicon (9th ed., 1940)
Pape
Hom. u. Aesch., für ἀνά, vor Lippenbuchstaben, ἂμ βωμοῖσι, ἂμ μέσον, ἂμ πεδίον, ἂμ φυτά.
Pape, Griechisch-deutsches Handwörterbuch (3. Aufl., 1914)