Umar bin Ali bin Husayn (may Allah have mercy on him) said: The Messenger of Allah (peace and blessings be upon him) ordered those skulls to be placed in the crops because of the evil eye. In another narration: The Messenger of Allah (peace and blessings be upon him) came to Madinah and said: "O Quraysh, you love livestock, so reduce it, for you are in a land with little rain; and cultivate, for cultivation is blessed, and place many skulls in it."
The first report is munqati' (broken), and this one too is mursal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المزارعة / 11753