Although AMA doesn't directly provide advice about Excel spreadsheets, you should apply the following, using information you can find about the author, title of the dataset, owner of the data and a URL or DOI.
According to the AMA Manual of Style: 3.11.4.2.2 Data Repository.
When citing data from a repository, cite the original source for the data.
1. Cutter AD, Gray JC. Data from: Ephemeral ecological speciation and the latitudinal biodiversity gradient. Dryad Digital Repository. Deposited August 17, 2016. doi:10.5061/dryad.734v9
The data package or data set should be cited in the original publication to link the publication and the data. Examples of citations to a data package follow:
2. Francuzik W. Data from: Skin microbiome in atopic dermatitis: 16S gene sequence data. figshare. 2016. doi:10.6084/m9.figshare.4028943
3. Levy I, Maor Y, Mahroum N, et al. Data from: Missed opportunities for earlier diagnosis of HIV in patients that presented with advanced HIV disease: a retrospective cohort study. Dryad Digital Repository. 2016. doi:10.5061/dryad.73c003.11.5