Abraham the Patriarch

Abraham's story is in Genesis 12-25 (cached).  He appears in a number of common narrative types. 

One is the meeting of Abraham and Melchizedek, popular because it was said to foreshadow the Eucharist. Indeed, the Melchizedek mosaic in Sant'Apollinare in Clase associates the latter's sacrifice with those of Abel and Abraham, all of them being types of Christ's sacrifice, of which the Eucharist is the memorial.

Another type is the "Hospitality of Abraham" -- when he was visited by three men who were taken by Christians to be God himself in the Trinity (see the page for the Trinity). 

Another narrative type with Eucharistic import is the Sacrifice of Isaac, Abraham's response to God's demand that he sacrifice his son, which was taken to be a foreshadowing of the sacrifice of Christ on the cross.  Abraham's sacrifice provides the elements for the portrait of Abraham at right: Isaac himself, the sword, and the fire used for the sacrifice.

Even as early as the beginning of the 3rd century, the art emphasizes the connection between Abraham's sacrifice and the remembrance of Christ's sacrifice in the liturgy (example). 

Except for some of the images with Melchizedek, Abraham has a white or gray beard slightly forked at the bottom edge, which sometimes reaches as far as mid-chest.

Feast day:
In his translation / expansion of the Golden Legend, Caxton notes that Abraham's legend was "read in the church" on Quinquagesima Sunday.

At left, 15th century painting in the Metropolitan Museum

Abraham images among the 5th century mosaics in Santa Maria Maggiore:
Other images of the Sacrifice of Isaac:
4th century sarcophagus
11th century a portable altar
11th century capital

Other images of Abraham and Melchizedech:
16th century Abraham and Melchizedech

Portrait Statue of Abraham alone:
Gian Maria Morlaiter's Statue in the Gesuati, Venice